One step in the fabrication of semiconductor integrated circuits is the formation of a conductor pattern over a semiconductor substrate through photolithographic masking and etching. This step typically requires the formation of a continuous sheet of metal film that is insulated from the substrate by a dielectric layer. A photoresist coating over the metal layer is selectively exposed to actinic light through a mask which defines the desired conductor pattern. The photoresist film is developed so that it in turn constitutes a mask having openings defining the desired conductor pattern. Modern integrated circuit techniques then often employ reactive ion etching for selectively etching away the exposed metal to the dielectric layer, and, after that, removal of the remaining photoresist leaves the desired conductor pattern overlying the dielectric layer.
Trends toward increased circuit density require a higher degree of control of the photolithographic processes to meet more stringent requirements for conductor pattern definition. Spurious reflection of the actinic light from the metal film has a tendency to blur the edges of the patterns being defined. A dyed photoresist can be used to reduce the effects of such reflections, but such compositions are dependent on thickness, shelf life and formulation, and thus do not give dependably uniform results. It has therefore been recognized that a separate antireflection coating should often be included between the metal layer and the photoresist film in integrated circuits made to a high degree of precision.
Various compositions that have been proposed for use as photolithographic antireflection coatings include titanium nitride, titanium-tungsten, silicon nitride and amorphous silicon. We have found that, particularly for use with gallium arsenide integrated circuits using aluminum conductors, these various known antireflection coatings have distint drawbacks. For example, titanium nitride has a very high stress when coated on aluminum, which may result in adhesion problems and other problems. Silicon coatings tend to react with aluminum and have antireflection properties that are highly dependent on thickness. There is, therefore, a well understood need in the industry for an antireflection coating that is robust in the sense that extreme care in controlling its characteristics need not be made, is chemically consistent with the use of metals such as aluminum and with subsequent device processing steps such as reactive ion etching, is able to adhere well to materials such as aluminum, and whose use is consistent with other processing requirements for making integrated circuits, especially gallium arsenide intergrated circuits.